Biodiesel Production Principles and Processes
Introduction
The process to make biodiesel involves a chemical reaction. This means
that the biodiesel industry is a chemical industry. Those involved in mak-
ing biodiesel must have a good understanding of the underlying chem-
istry to ensure they are making quality fuel in a safe manner.
Biodiesel is an alternative fuel for diesel engines that is produced by
chemically reacting a vegetable oil or animal fat with an alcohol such as
methanol or ethanol. In words, the reaction is:
Oil + alcohol → biodiesel + glycerin
The photo shows a bottle of biodiesel and glycerin (also called glycerol).
The biodiesel is the lighter-colored layer at the top. The darker-colored
crude glycerin has settled to the bottom.
Bottle of biodiesel (top layer) and glycerol (bottom layer)
It is important to realize that unmodified vegetable oil, sometimes called
straight vegetable oil (SVO) or waste vegetable oil (WVO), is not
biodiesel. Some people have used SVO or WVO in diesel engines with
varying degrees of success. The primary problem is the high viscosity
and low volatility of the unmodified vegetable oils. Without exception,
U.S. engine manufacturers have recommended against the use of SVO
and WVO.
Biodiesel is usually preferred over SVO and WVO because the chemical
reaction converts the oil or fat into compounds that are closer to the hy-
drocarbons found in regular diesel fuel.
The chemical reaction that converts a vegetable oil or animal fat to
biodiesel is called "transesterification." This is a long name for a simple
process of combining a chemical compound called an "ester" and an al-
cohol to make another ester and another alcohol. Oils and fats are in-
cluded in the ester family. When they react with methanol or ethanol,
they make methyl or ethyl esters and a new alcohol called glycerol or,
more commonly, glycerin.
The vegetable oils and animal fats used to make biodiesel can come
from virtually any source. All of these products consist of chemicals
called triglycerides, so biodiesel can be made from soybean oil, canola
oil, beef tallow, and pork lard, and even from such exotic oils as walnut
oil or avocado oil.
Even used cooking oil or waste oil can be used to make biodiesel. How-
ever, these oils present special challenges for biodiesel production be-
cause they contain contaminants such as water, meat scraps, and
breading that must be filtered out before the oil is converted to biodiesel.
Methanol is the most common alcohol used for making biodiesel. It is
sometimes called methyl alcohol or wood alcohol. It is very toxic, and
swallowing as little as a spoonful can cause blindness or even death.
Dangerous exposure can also occur from breathing methanol vapors or
absorbing methanol through skin contact. In the United States, ethanol
is usually more expensive than methanol, so it is used less frequently to
make biodiesel. It is the alcohol that is found in alcoholic drinks, so it is
not toxic in small amounts. However, it is subject to very challenging
government regulations because of the tax requirements associated with
alcoholic beverages.
The chemical reaction used to make biodiesel requires a catalyst. A cat-
alyst is usually a chemical added to the reaction mixture to speed up the
reaction. Since the catalyst is not consumed in the reaction, it will be left
over at the end in some form. In biodiesel production, the actual com-
pound that catalyzes the reaction is called methoxide. One common way
to make methoxide is to dissolve sodium hydroxide or potassium hy-
droxide in methanol. Large producers buy a solution of sodium methox-
ide in methanol that is much safer to work with.
High-quality biodiesel is defined by compliance with the American Soci-
ety for Testing and Materials (ASTM) specification D6751. Fuel testing to
verify compliance can be expensive, especially for small producers, but
it is the most reliable way to ensure that fuel consumers will have access
to high-quality fuel.
